1. Field of the Invention
The present invention relates to active structural acoustic control (ASAC) of sound transmission through and radiation from structures. The invention is directed to a method and apparatus implementing the method which derives acoustic far-field information from the vibrational field exhibited by a structure, thus enabling the use of vibration (error) sensors and vibration control actuators mounted on or embedded in the structure, and which is applicable to any structure and environment boundary conditions.
The method can be used to control noise in the cabins of jet-driven and propeller-driven aircraft, helicopters and automobiles, as well as to control machinery noise and in other industrial noise control applications.
2. Description of the Related Art
The need for reducing sound radiated from and transmitted through elastic structures such as aircraft cabin panels is a common problem. Attempts at achieving moderate levels of noise reduction by passive applications and structural modifications have been inadequate for lower frequencies. The possibility of using active devices for this type of noise control is evolving in conjunction with the development of high-speed microprocessors, with the objective of developing intelligent structures that can adapt to their environment such that undesired vibration and acoustic radiation can be minimized.
In experimental work published to date, an adaptive controller operating in the time-domain according to a least-mean-square (LMS) algorithm typically is used to provide required control signals to single or multiple control actuators attached to a vibrating structure. An estimate of one or multiple response variables, so-called error signals, is needed for the adaptive controller to operate and minimize the response of the structure. The LMS adaptive algorithm principle was first described by B. Widrow et al. in Proceedings of the IEEE, Vol. 63, No. 12 (1975), pp. 1692-1716, and generalized to multiple error sensors by S. J. Elliott et al. in IEEE Transactions on Acoustics, Speech and Signal Processing ASSP-35(10) (1987), pp. 1423-1434.
One ASAC method, described by V. L. Metcalf et al. in Journal of Sound and Vibration, Vol. 153, No. 3 (1992), pp. 387-402, uses discrete vibration transducers fixed to a structure, such as accelerometers, to provide error signals which correspond to the total response (dominated by resonant vibrations) of the structure. This method has proven ineffective in controlling the non-resonant (or "acoustically fast") components of vibrational energy which are responsible for sound radiation and transmission. Use of discrete vibration transducers can cause spillover of vibrational energy which increases vibration levels of a structure without effecting any significant reduction in sound radiation. Such spillover is primarily due to coupling between point-force actuators and the structure, resulting in excitation of additional vibrational modes.
Another ASAC method, described by V. L. Metcalf et al., supra, uses microphones positioned in the radiated acoustic far-field as the error sensors. Although good noise attenuation results have been achieved, in many situations the use of far-field microphones may not be desirable or even feasible. To develop a true adaptive structure, the sensing must become an integral part of the structure; therefore, the microphones located in the far field must somehow be eliminated.
Yet another ASAC method, described by R. L. Clark and C. R. Fuller in Journal of the American Acoustical Society, Vol. 91, No. 6 (1992), pp. 3321-3329, uses polyvinylidene fluoride (PVDF) piezoelectric films bonded to the surface of a structure as error sensors. The use of distributed transducers such as PVDF films is not useful in many practical applications. For example, for aircraft noise control it is not feasible to put large PVDF films on structural panels due to many practical difficulties. An aircraft fuselage contains numerous stiffeners, frames and rivets, leaving only small, exposed areas on which PVDF film could be mounted. Thermal and acoustic blankets, compressed between the fuselage skin and interior trim panels, will interfere with PVDF film. PVDF films are not strong and rugged and are therefore susceptible to damage during fuselage manufacturing and assembly, and are subject to performance degradation in harsh operating environments. In addition, because PVDF films are distributed sensor arrays, the complexity inherent in their design generally requires assuming simplified boundary conditions which may render the films ineffective in practical use.
In view of the limitations of ASAC methods known in the art, there is a need for a method enabling error sensors and control actuators to be mounted on or embedded in a structure, so that both sensing of the vibrational field and control of the radiated and transmitted acoustic field can be accomplished by means integral to the structure. Ideally, the method should not require a large amount of energy from the control actuators, nor be limited in applicability to specific structure and environment boundary conditions.